Traditional ultrasound image diagnostic apparatuses inspect the interiors of subjects through emitting ultrasound waves into the subjects to receive ultrasound waves (echoes) reflected from the subjects, and conducting predetermined processes to signal data on the received echoes. Such ultrasound image diagnostic apparatuses are used for various purposes, such as medical tests and treatments and inspection of the internal structures of buildings.
The ultrasound image diagnostic apparatuses can generate images having good contrast from the harmonic components (e.g., frequencies 2f0 and 3f0) of transmitting signals relative to their fundamental components (frequency f0). This technique is called tissue harmonic imaging (THI).
These harmonic components are mainly caused by the non-linear distortion of ultrasound waves during propagation in a subject. In specific, incident ultrasound waves in a living body are distorted by the non-linear response of tissues during propagation in the tissues, resulting in increased harmonic components. The response signals corresponding to the ultrasound waves thus include, for example, components of frequencies 2f0 and 3f0 (two and three times the frequency f0 of fundamental waves).
The harmonic components in the tissue harmonic imaging can be extracted by a known method, such as a filtering technique or pulse inversion technique.
In the filtering technique, a band-pass filter having a center frequency of 2f0, for example, extracts 2f0 harmonic components from response signals. In the pulse inversion technique, the apparatuses transmit first and second pulse signals that are polarity-inverted or time-inverted at a predetermined time interval, and combine their response signals to offset the fundamental components, thereby emphasizing the second harmonic components.
A typical ultrasound image diagnostic apparatus based on the pulse inversion technique generates fundamental image signals from first echoes after the first transception (transmission and reception) of fundamental ultrasound waves, generates harmonic image signals through addition of second echoes after the second transception of inverted fundamental ultrasound waves to the first echoes, and then combines the fundamental image signals with the harmonic image signals, which are weighted depending on the depth for each line (scanning line) (refer to Patent Literature 1). The synthesized image is made of a harmonic image in a shallow portion and a fundamental image in a deep portion, and has few artifacts and sufficient sensitivity even in the deep portion.
The use of the ultrasound image diagnostic apparatuses is not limited to ultrasound image display of a subject based on the processed data on received echoes. The ultrasound image diagnostic apparatuses are also used to identify and visualize the position of a puncture needle relative to a specific portion (target) in a subject during the inserting of the puncture needle into the target for sampling the target, discharging water from the target, or injecting or indwelling an agent or marker into the target. Such ultrasound image can achieve rapid, certain, and ready treatment for the target in the subject.
The pulse inversion technique can be applied to the imaging of a subject using such a puncture needle. For example, a typical ultrasound image diagnostic apparatus repeats the first transception of positive ultrasound waves, the second transception of negative ultrasound waves (inverted from the positive ultrasound waves), and the third transception of negative ultrasound waves; generates a normal mode image through combining the first echoes and the second echoes; and generates a puncture mode image through combining the first echoes and the third echoes (refer to Patent Literature 2). The normal mode image is based on a relatively low pulse repetition frequency (PRF) and thus has reduced motion artifacts caused by motions of the living body, whereas the puncture mode image is based on a relatively high PRF and can thus capture motion artifacts caused by motions of the puncture needle well.